Tiny
hole guides atoms against tide

By
Kimberly Patch,
Technology Research NewsOne of the most elegant and important processes
of life is the ion channel, which underlies the nerve signals that carry
communications throughout our bodies.

Nerve cells transmit signals to other nerve cells by allowing positively-charged
atoms, or ions, of sodium, potassium and calcium on the outside of a neural
membrane to switch places with negatively-charged chloride ions on the
inside. This depolarizes the membrane, releasing the energy stored in
the original arrangement in order to signal an adjoining neuron.

The trick is getting the ions to flow back so the nerve cell can do it
again. Nature uses chemistry to coax the ions to make the return trip
against their electrochemical potential through special pores, or channels,
within the membrane.

Researchers from the Silesian University of Technology and Jagellonian
University in Poland have made a synthetic device that uses an electrical
field and an extremely small, conical pore in a thin film of material
to coax potassium ions through the artificial membrane against their electrochemical
potential.

The device can be used to study and better understand the biological ion
pump. It could also eventually be used to power microscopic machines.

It was already known that cone-shaped pores that are as small as molecules
produce an asymmetric electrical effect similar to a cell membrane's ion
pump. The researchers proved, however, that this effect could be used
to pump ions as well. "[We thought] that perhaps our conical pore could
work according to the same principle," Zuzanna Siwy, an assistant professor
at the Silesian University of Technology in Poland and a guest scientist
at the Institute for Heavy Ion Research (GSI) in Germany.

The device works by ratcheting the molecules through the widening, and
therefore sloping channel, said Andrzej Fulinski, a professor of physics
at Jagellonian University in Poland. The oscillating, or periodic electric
field drags the ion to and fro, said Fulinski. The net effect is that
ions are pushed through the channels and out the wide side of the pore,
concentrating the ions on that side of the device.

A key to making the device pump ions against their natural direction is
that once they enter the cone-shaped channel, it is easier to go down
the widening sloping of the cone than up the walls of the channel. "It
is easier to go uphill along a less steep slope," said Fulinski. This,
together with friction, leads to the pumping effect. "This is the principal
on which both [the] pump and molecular motors, or ratchets work," he said.

The challenge was making a conical pore small enough for ions. The researchers'
pore had an opening that widened from two nanometers to 500 nanometers.
A nanometer is one millionth of a millimeter, and two nanometers is the
width of 20 hydrogen atoms.

To fabricate such a small opening, the researchers bombarded a tiny bit
of polymer, or plastic, film with a high-energy ion beam, then chemically
etched the remainder of the tapered hole.

When the researchers put the device in a salt solution, they found that
more potassium ions flowed from the narrow toward the wide opening of
the cone, increasing the concentration of ions on that side of the plastic
membrane. The reaction kept going even when there was a 100-fold concentration
difference between the two sides of the membrane, according to Siwy.

The researchers found, however, that as concentration changes, the reaction
gets less efficient in terms of the energy used for the field per ion
pumped. The method is 40 percent efficient when the concentration of ions
is the same on both sides of the membrane, and drops to 10 percent when
the concentration is 7.5 to 1.

The pumping phenomenon is determined by the size of the narrow side of
the pore, the surface change of the cone, and the frequency of the alternating
electrical field, according to Siwy. When the narrow side of the cone
is 15 nanometers or larger, the reaction does not work.

The researchers originally made the device in order to study synthetic
models of biological ion channels, said Fulinski. "These enable measurements
which are impossible to perform on living material," he said.

While constructing the synthetic pores, however, [Siwy] realized that
the electrical characteristic of the pores would allow them to pump ions
using an electric field, said Fulinski. "The measurements... confirmed
the suggestion, and we were able to show that indeed such a device works,"
he said.

The pump can be used as a sort of diode that works in a watery environment,
said Fulinski. An electrical diode guides current in only one direction.
"The pump can be viewed as a rectifier of ionic currents," he said.

The next step in the research is to make the pump work faster and more
efficiently, said Siwy. The researchers are looking to decrease the length
of the pore, which is currently 1,000 times longer than biological pores.
It will take at least two years before the pump can be used in practical
devices, Siwy said.

Siwy and Fulinski published the research in the November 4, 2002 issue
of Physical Review Letters. The research was funded by the Alexander Von
Humboldt Foundation in Germany, The Institute for Heavy Ion Research (GSI)
and the Foundation for Polish Science.